The present invention relates to photodiodes, and more particularly to a photodiode exhibiting an increased ON/OFF current ratio and a method for fabricating the same.
Photodiodes are used for contact image sensors, bar code readers, facsimiles and etc.
FIG. 1 is a sectional view of a conventional photodiode using chromium silicide as its upper electrode.
For fabricating the photodiode shown in FIG. 1, a chromium film 12 is deposited in a vacuum over a glass substrate 11. The chromium film 12 is patterned by use of the wet etching process to form a lower electrode.
Thereafter, a hydrogenated amorphous silicon (a-Si:H) film 13 is deposited to a thickness of 1 .mu.m over the entire exposed surface of the resulting structure. Over the a-Si:H film 13, another chromium film (not shown) is deposited. Subsequently, the chromium film is partially etched. Using the etched chromium film as a mask, the a-Si:H film 13 is patterned by use of the reactive ion etching (RIE) process.
The resulting structure is then subjected to an annealing treatment at a temperature of 250.degree. C. in a N.sub.2 atmosphere for one hour. By this annealing treatment, chromium silicide 14 is formed on an interface between the chromium film and the a-si:H film 13, thereby providing an upper electrode.
Using an (NH.sub.4).sub.2 Ce(NO.sub.3).sub.6 acid, the remaining chromium film not forming silicide is removed. Thus, a photodiode is fabricated.
When a "+" bias is applied to the chromium film 12 while a "-" bias is applied to the chromium silicide 14 in the photodiode having the above-mentioned structure, a potential difference is generated due to the applied "+" bias and "-" bias. As a result, a current flows from the chromium film 12 as the lower electrode to the chromium silicide 14 as the upper electrode.
A Schottky effect is generated at the interface between a-Si:H film 13 and the chromium silicide 14. Due to such a Schottky effect, a Schottky barrier is formed which limits the dark current to several tens picoamperes.
Upon receiving light, the a-Si:H film 13 generates such as large amount of photo charges that the Schottky barrier no longer serves as the barrier for photo charges. As a result, a large amount of current flows through the interface between Che a-Si:H film 13 and the chromium silicide 14.
However, the fabrication of the above-mentioned conventional photodiode is complex because the chromium film deposited on the a-Si:H film 13 for the formation of the chromium silicide 14 should be removed after the formation of the chromium silicide 14.
FIG. 2 is a sectional view of another conventional photodiode using a transparent conduction film as its upper electrode.
For fabricating the photodiode shown in FIG. 2, a chromium film 22 is deposited over a glass substrate 21 to form a lower electrode. Thereafter, a hydrogenated amorphous silicon (a-Si:H) film 23 is deposited over the entire exposed surface of the resulting structure by using a plasma enhanced chemical vapor deposition (PECVD)process. Over the a-Si:H film 23, a transparent conduction film 24 made of indium thin oxide is deposited using a the sputtering process. Subsequently, the transparent conduction film 24 and the a-Si:H film 23 are etched using an RIE process to form an upper electrode and a photo conduction layer, respectively.
FIGS. 3A to 3C are diagrams respectively illustrating energy bands depending on voltages applied to the electrodes of the photodiode shown in FIG. 2.
When no voltage is applied to the photodiode, no current flows through the photodiode, as shown in FIG. 3A.
When a forward voltage is applied to the photodiode, that is, when "-" voltage is applied to the chromium film 22 as the lower electrode while "+" voltage is applied to the transparent conduction film 24, a large amount of dark current flows from the transparent conduction film 24 to the lower electrode 22 because a Schottky barrier is formed at an interface between the photo conduction layer 23 and the transparent conduction film 24, as shown in FIG. 3B.
As a result, a large amount of photo current can flow because of no effect of the Schottky barrier. At the forward bias state, it is, therefore, impossible to obtain a high photo current/dark current ratio.
On the other hand, when a reverse voltage is applied to the photodiode, that is, when "+" voltage is applied to the chromium film 22 as the lower electrode while "-" voltage is applied to the transparent conduction film 24, no current flows because a high potential barrier is formed between the photo conduction layer 23 and the transparent conduction film 24, as shown in FIG. 3C. As a result, little dark current flows.
As the photodiode is exposed to light under the condition that the reverse voltage is applied to the photodiode, a large amount of photo charges, namely, electrons and holes are generated in the photo conduction layer 23.
The generated electrons flow toward the lower electrode 22 while the generated holes flow the transparent conduction film 24. As a result, photo current can flow from the lower electrode 22 to the transparent conduction film 24.
Where the reverse voltage is applied to the photodiode, as mentioned above, the current flow through the photodiode is determined by the intensity of light applied to the photodiode.
The conventional photodiode shown in FIG. 2 can obtain a high ON/OFF current ratio in that the transparent conduction film 24 and the a-Si:H film 23 form the Schottky barrier inhibiting a flow of dark current at the reverse bias state. However, it is difficult to form a satisfactory Schottky barrier because the composition of the transparent conduction film 24 may be easily varied depending on the process used in depositing of the transparent conduction film 24 as the upper electrode and because the interface between the transparent conduction film 24 and the a-Si:H film 23 may be unstable.
As a result, the conventional photodiode has a problem that unstable dark current is generated between the photo conduction layer 23 and the transparent conduction film 24.
FIG. 4 is a sectional view of a photodiode having a conventional PIN structure. On the other hand, FIG. 5 is a diagram illustrating an energy band in the photodiode of FIG. 4.
As shown in FIG. 4, the photodiode comprises a lower electrode 31, a photo conduction layer 32 formed over the lower electrode 31, a transparent conduction film 33 formed over the photo conduction layer 32, and a metal electrode 34 formed over the transparent conduction film 33. In this structure, "+" voltage is applied to the lower electrode 31 while "-" voltage is applied to the transparent conduction film 33 as an upper electrode via the metal electrode 34.
The photo conduction layer 32 has a PIN structure including a high concentration n.sup.+ a-Si:H film 32-1, an intrinsic a-Si:H film 32-2 and a high concentration p+a-Si:H film 32-3.
As the upper electrode 33, a chromium silicide film may be used in place of the transparent conduction film.
It is preferred that the upper electrode 33 adapted to pass light therethrough has a high transparency and a high conductivity. To this end, the transparent conduction film 33 comprised of an ITO film is bonded with the metal electrode 34 exhibiting a high conductivity to constitute a signal line because the ITO film exhibits a low conductivity.
This conventional photodiode having the above-mentioned structure achieves a signal sensing function obtained from the current difference between a case in which the photodiode is exposed to light and a case in which the photodiode is exposed to no light, under a reverse bias state, that is, when "+" voltage is applied to the lower electrode 31 while "-" voltage is applied to the upper electrode 33.
The current generated when the photodiode is exposed to light is called an ON current I.sub.photo whereas the current generated when the photodiode is exposed to no light is called an OFF current or dark current I.sub.dark.
When the photodiode is exposed to light having an energy higher than an energy band gab Eg which is the difference between a conduction band Ec and a valence electron band Ev, as shown in FIG. 5, photo carriers are generated. As the photo carriers move, a flow of current is generated. This current is the photo current I.sub.photo.
Where the energy band gap Eg is small, photo carriers are generated even if the photodiode is exposed to light having an energy lower than the energy band gab Eg. To this end, it is required to shield 100% of the light so as to accurately distinguish the photo current from the dark current.
However, it is difficult to perfectly shield the light. As a result, dark current is generated.
In photodiodes utilized in facsimiles for sensing light reflected from a document or other objects and thus recognizing characters and letters, the accurate control for dark current is very important for gray scale level designation. Where the level of dark current is imperfectly determined, the sensing operation is inaccurately achieved.
Where a plurality of photodiodes are used for a large document including a white part at one portion thereof and a black part at the other portion thereof, the characteristics of the photodiodes should be uniform.
For obtaining the accuracy in the sensing operation and the uniformity in device characteristic, a material exhibiting an appropriate energy band gap should be used. Furthermore, such a material exhibiting the appropriate energy band gap must be able to maintain the uniformity in device characteristic and control the dark current.
On the other hand, where the energy band gap is too large, it is impossible to accomplish the sensing function because all signals generated are detected as dark current.
Therefore, a good characteristic can be obtained in photodiodes wherein a large amount of photo current can flow while controlling a flow of dark current, thereby obtaining a high I.sub.photo /I.sub.dark ratio.
The intrinsic a-Si:H film 32-2 exhibits an energy band gap Eg of about 1.7 eV. Where a trap is present, photo carriers are generated even by a light energy lower than the energy band gap Eg, as mentioned above. Of course, such a problem does not occur when the intrinsic a-Si:H film has a good quality. When photo carriers are generated by the light energy lower than the energy band gap Eg, an amount of dark current is increased, thereby resulting in a decrease in I.sub.photo /I.sub.dark ratio. As a result, the photodiode exhibits a decreased signal/noise (S/N) ratio.